Engines of construction machinery such as bulldozers and hydraulic excavators are cooled by circulating cooling water (a coolant), heat produced by the engine being dissipated when the cooling water passes through a radiator. With construction machinery, unlike with an automobile or the like, there is little opportunity for an air current caused by traveling to strike the radiator, and hence it is necessary to constantly rotate a hydraulically driven cooling fan in a forward rotational direction, thus creating an air current passing over the radiator so as to bring about heat dissipation. Note that there are also models of construction machinery having a configuration in which the hydraulically driven cooling fan is rotated so as to create an air current passing over an oil cooler, thus dissipating heat from hydraulic operating oil. In this case, the oil cooler and the radiator are installed in order along the path of the air current created by the hydraulically driven cooling fan.
In the case that the hydraulically driven cooling fan is used purely to cool the cooling water and/or operating hydraulic oil in this way, a fan that can only be rotated in the forward rotational direction may be used.
However, upon the radiator or oil cooler being used for a long time, clogging with rubbish may occur, and hence the cooling performance may be impaired.
Accordingly, from hitherto, hydraulic circuitry as shown in FIG. 6 has been constructed, so that rubbish can be removed using the hydraulically driven cooling fan. In FIG. 6, the oil cooler is omitted, a configuration in which only the radiator is cooled being shown.
That is, as shown in FIG. 6, there are provided a hydraulic pump 18 that is driven by an engine 4, a hydraulic motor 15 that is driven by hydraulic oil ejected from the hydraulic pump 18 and rotates in a forward rotational direction or a reverse rotational direction in accordance with the direction of the supplied hydraulic oil, a hydraulically driven cooling fan 13 that is driven by the hydraulic motor 15, and a switching valve 220.
Upon the switching valve 220 being switched to a forward rotation position, the hydraulic oil ejected from the hydraulic pump 18 is supplied via an oil line 19a and the switching valve 220 to a port MA of the hydraulic motor 15, and is discharged from a port MB of the hydraulic motor 15 via the switching valve 220 and an oil line 19b into a reservoir 21. Consequently, the hydraulic motor 15 rotates in the forward rotational direction, and hence the hydraulically driven cooling fan 13 rotates in the forward rotational direction. As a result, an air current cooling a radiator 12 is created, and hence heat is dissipated from cooling water passing through the radiator 12.
On the other hand, upon the switching valve 220 being switched to a reverse rotation position, the hydraulic oil ejected from the hydraulic pump 18 is supplied via the oil line 19a and the switching valve 220 to the port MB of the hydraulic motor 15, and is discharged from the port MA of the hydraulic motor 15 via the switching valve 220 and the oil line 19b into the reservoir 21. Consequently, the hydraulic motor 15 rotates in the reverse rotational direction, and hence the hydraulically driven cooling fan 13 rotates in the reverse rotational direction. As a result, an air current blowing out rubbish from the radiator 12 is created, and hence rubbish clogging the radiator 12 is blown out.
However, in a state in which the hydraulic oil is being ejected from the hydraulic pump 18 into the oil line 19a at a large flow rate and a high pressure so that the hydraulically driven cooling fan 13 is rotating at a high rotational speed, if the switch position of the switching valve 220 is reversed, then cavitation arises in the oil line during the switching, and hence the peak pressure of the hydraulic oil flowing through the oil line rises. As a result, the hydraulic equipment is subjected to an excessive load, which may affect the durability of the hydraulic equipment. Moreover, the hydraulically driven cooling fan 13 reverses while maintaining a high rotational speed, and hence the fan produces much noise upon the reversal, giving an operator an unpleasant or incongruous feeling. An abnormal noise may also be produced by other hydraulic equipment upon the reversal, again giving the operator an unpleasant or incongruous feeling.
The higher the rotational speed of the engine 4, and hence the higher the rotational speed of the hydraulically driven cooling fan 13, or the lower the oil temperature, the higher the peak pressure becomes, and hence the greater the effect on the durability of the hydraulic equipment, and the greater the effect on the operator.
To prevent this situation, various art for reducing the peak pressure produced upon reversing the switch position of the switching valve has thus been proposed from hitherto.
(Prior Art Seen in Patent Document)
A cited patent document is Japanese Patent Application Laid-open No. 2002-349262.
(Prior Art 1)
First, in the “Problem to be Solved” section in the patent document, there is described an invention in which the switching valve 220 shown in FIG. 6 is constructed as a 2-position switching valve having a forward rotation position and a reverse rotation position but not having a neutral position, and when the switch position of the switching valve 220 is reversed, the engine 4 and the hydraulically driven cooling fan 13 are temporarily stopped.
(Prior Art 2)
In the “Working Examples” section in the patent document, there is described an invention in which the switching valve 220 shown in FIG. 6 is constructed as a 2-position switching valve having a forward rotation position and a reverse rotation position but not having a neutral position, in addition to this switching valve 220 there is separately provided a rotation-stopping switching valve for stopping the rotation of the hydraulically driven cooling fan 13, and when the switch position of the switching valve 220 is reversed, the rotation-stopping switching valve is switched, so as to temporarily stop the rotation of the hydraulically driven cooling fan 13.
(Prior Art 3)
Furthermore, in the “Working Examples” section in the patent document, there is described an invention in which the switching valve 220 shown in FIG. 6 is constructed as a 3-position switching valve provided with a neutral position in which the oil line 19a and the oil line 19b are communicated together (a fan-stopping position) between the forward rotation position and the reverse rotation position, and when the switch position of the switching valve 220 is to be reversed, the switching valve 220 is first positioned in the neutral position (the fan-stopping position), so as to temporarily stop the rotation of the hydraulically driven cooling fan 13.
According to prior art 1 described above, the engine 4 stops each time the switch position of the switching valve 220 is reversed, and hence each time an operation of restarting the engine 4 is required. Operation is thus burdensome for the operator, and moreover the work efficiency is greatly impaired.
According to prior art 2 described above, there is no need to stop the engine 4 each time the switch position of the switching valve 220 is reversed, and hence the problem of prior art 1 is resolved; however, a rotation-stopping switching valve must be provided in addition to the switching valve 220, and hence existing hydraulic circuitry must be modified, and the apparatus cost increases.
According to prior art 3 described above, there is no need to stop the engine 4 each time the switch position of the switching valve 220 is reversed, and hence the problem of prior art 1 is resolved; however, the switching valve 220 must be constructed as a 3-position switching valve, for which the construction of the valve itself and a control apparatus is more complex than for a 2-position switching valve, and hence the apparatus cost increases.